Switching power supplies are used in numerous electronic devices to generate the low DC voltage required to supply the electronic components with power from a line voltage. Here, in many applications, switching power supplies have become popular compared with conventional line supply circuits with power transformers because they provide better efficiency above a certain power class, and in particular they have smaller space requirements. This is because, instead of using the line voltage, a high-frequency AC voltage is transformed, which can be in the frequency range of 20 kHz to 200 kHz, for example, instead of the 50 Hz or 60 Hz of the line voltage. Because the required number of transformer windings decreases in inverse proportion to the frequency, the I2R losses can be greatly reduced and the actual transformer becomes significantly smaller.
Such switching power supplies have a primary side and a secondary side, wherein the transformer has a primary-side winding and a secondary-side winding. A primary-side switch is connected to the primary-side winding in order to interrupt a flow of current through the primary-side winding, and the switching power supply has a free-running circuit for generating switching pulses that trigger the primary-side switch.
In order to optimize the degree of efficiency, in particular, primary-switched switching power supplies are known in which the frequency generated on the primary side of the high-frequency transformer by the switch, for example, a bipolar transistor, is regulated as a function of the load applied to the secondary side of the line voltage supply circuit in order to regulate the transmitted power. The feedback required for such regulation is realized in an especially simple way, such that an additional, primary-side auxiliary winding is arranged on the transducer, wherein an indicated voltage is generated by this auxiliary winding and wherein this indicated voltage indicates the voltage to be regulated on the secondary side with the aid of the auxiliary winding on the primary side. The voltage tapped on the auxiliary winding can then be used as a regulating parameter. The known regulating circuits, however, are usually either relatively expensive in terms of components or demand tight tolerances of the electronic components that are used, in order to achieve the necessary regulating accuracies.
Therefore, the problem of the present invention is to disclose a trigger circuit for the switch in a switching power supply and also a corresponding switching power supply that allows, with reduced complexity and relatively wide tolerances in the electronic components that are used, improved regulation characteristics and increased flexibility with respect to the operating parameters.